The Iowa Department of Natural Resources says the drought has caused Big Creek Lake north of Des Moines to drop more than 6 feet below normal levels.
The DNR says the lake continues to fall by 6 inches a week.
The lake's low level has helped crews installing new boat ramps by allowing workers to pour the concrete slabs in place rather than on shore.
But Big Creek State Park Manager Chad Kelchen says soon the lake will be so low that no boat ramps will be usable.
Officials also worry that a water main that normally is under about 9 feet of water could be damaged because it's now exposed.
The lake was created by a diversion dam built as part of efforts to protect Polk City from floods.